WoW is not a totally free market, just like it's not totally wide-open combat, even on a PvP server. WoW is a moderated market - there are tools in place to help people to be careful, and Blizzard has some control over the base money and item supply. Blizzard also sets the vendor prices, but it doesn't set the AH prices.

Blizzard is also fairly forgiving of player goofs - I've gotten back items I'd accidentally vendored, and know people who've gotten back the characters they deleted months ago - but they're less forgiving of goofs between players, like mis-buying at the AH.

The forum QQing warrants barely any response, let alone a blue one. I'm wondering if Blizzard has gotten a bunch of complaints about player stupidity in the AH and finally posted something to clarify the obvious.

Finally, I've heard (and speculated) that putting simple items on the AH for large amounts of gold is one way gold is aquired from gold sellers - give them the money offline, and they buy your over-priced items online. That is about the only argument I can think of for limiting how much items can be offered for on the AH.